The present invention relates to filtration media for use in high efficiency air filters, for filtering particulate material from gas streams.
High efficiency particulate air (HEPA) level performance is critical to many of the processes in which HEPA filtration is used, such as semiconductor manufacturing and bio cleanrooms. In such applications, particulate material in the air is occasionally responsible for product defects or loss of containment of substances within the cleanroom. Instability of performance of the HEPA filters used in these applications is not acceptable.
Nonwoven filter elements can be used as surface loading media in HEPA filtration. The media are generally constructed to be permeable to gas flow, and to also have a sufficiently fine pore size and appropriate porosity to inhibit the passage of particles greater than a selected size therethrough. As particulate materials pass through the filter, the upstream side of the filter operates through diffusion and interception to capture and retain selected sized particles from the gas stream. The particles are collected to form a “dust cake” on the upstream side of the filter, and within the filter.
Currently, commercial HEPA filtration media fall into the categories of microglass fibers and blends of microglass, and electrostatically treated (also known as “electret”) high basis weight meltblown webs laminated to pleatable substrates. Each of these classes of filtration media has its limitations.
HEPA media utilizing microglass fibers and blends containing microglass fibers typically contain small diameter glass fibers arranged in either a woven or nonwoven structure, having substantial resistance to chemical attack and relatively small porosity. Such glass fiber media are disclosed in the following U.S. Patents: Smith et al., U.S. Pat. No. 2,797,163; Waggoner, U.S. Pat. No. 3,228,825; Raczek, U.S. Pat. No. 3,240,663; Young et al., U.S. Pat. No. 3,249,491; Bodendorfet al., U.S. Pat. No. 3,253,978; Adams, U.S. Pat. No. 3,375,155; and Pews et al., U.S. Pat. No. 3,882,135. Microglass fibers and blends containing microglass fibers have the limitation that they are typically relatively brittle when pleated, and produce undesirable yield losses as HEPA filters are required to be 100% inspected. Spot repairs are a tedious manual process.
Electrostatically treated meltblown webs, as described in U.S. Pat. Nos. 4,874,659 and 4,178,157, perform well initially, but are subject to performance drops as the media begin to capture particles and the electrostatic charge becomes insulated. U.S. Pat. Nos. 4,874,659 and 4,178,157 both disclose HEPA air filter media characterized by lower pressure drop in a range of 5-10 mm H2O (water column). These filters are made of nonwoven web (U.S. Pat. No. 4,874,659) or sliced films (U.S. Pat. No. 4,178,157) made of polyolefins, such as polyethylene or polypropylene, which are partially melted by heating to about 100° C. and are thereafter subjected to an electrical field which electrically charges the polymer. The charge imparted provides these filters with the high efficiency particulate air qualities. Such filters suffer a few limitations. First, being based on the electrical charge for effective capture of particulates, the performances of such filters are greatly influenced by air humidity, causing charge dissipation. Second, due to their mode of action and to being relatively thin, such filters are characterized by low dust load (the weight of dust per area of filter causing a two fold increase in pressure drop) per filter weight per area ratio of about 0.8, wherein typically the dust load of such filters is about 50-80 g/m2 and their weight per area is about 80-130 g/m2.
U.S. Pat. No. 6,604,925 discloses filtration media suitable for HEPA and a device and method for making the media by electrostatically spinning fine fibers through a nozzle to form a fibrous structure on a moving precipitation electrode, and then filling the pores of the fibrous structure with aerosol particles.
U.S. Patent Application Publication US 2004/0038014 A1 discloses filtration media including nanofiber layers for various applications including filtration of gas and liquids containing particle contaminants, and processes for making the media. PCT International Publication Numbers WO 02/20132 A2 and WO 02/20133 A2 disclose filtration media including low basis weight layers of nanofibers for various applications including filtration of air containing particle contaminants, and processes for making the media. These publications do not disclose high efficiency particulate air filter media or a process for achieving high efficiency air filtration.
It would be desirable to provide a means for achieving HEPA level air filtration while avoiding the above-listed limitations of known filtration media.